2. Review: What is an isotope?

3. Using Isotopes to Understand Photosynthesis Lab: Formulating Models

5. C. B. van Neil (1897-1985) demonstrated that photosynthesis is a light-dependent reaction in which the O 2 comes from water. Other scientists confirmed his findings by using radioactive isotopes of oxygen as tracers. Radioactive tracers are used to follow a particular molecule through a chemical reaction. C. B. van Neil Stanford University, Ca.

6. Procedure (Teacher Directed) 1. Check the blocks. One of one color = Carbon, four of another color = Oxygen, four of a third Color = hydrogen. Remove any stickers that remain on the blocks. 2. Study the following general equation that resulted from the van Neil experiment with photosynthesizing bacteria: CO 2 + 2H 2 O* -> CH 2 O + H 2 O + O 2 * Carbon dioxide + 2 water yields formaldehyde + water + oxygen gas His discovery predicted that H 2 O is the hydrogen donor in green plant photosynthesis and is oxidized to O 2. The chemical summation of photosynthesis was a milestone in the understanding of the chemistry of photosynthesis. Radioactive water, water tagged with an isotope of oxygen as a tracer (shown by the *), was used. Note where the tagged oxygen ends up on the right side of the equation.

7. Procedure Cont. 2. Set up van Neil’s experiment on the reaction template paper. Build the Carbon dioxide and the two water molecules place them in the appropriate place. The oxygen in the water molecules must be “tagged” with the sticker. It represents the isotope tracer in van Neil’s experiment. 3. Now use the same blocks to form the products of the van Neil experiment and make sure that the Oxygen isotopes form O 2 atmospheric oxygen.

8. Student Pairs (Your turn!) 1. Assume that the experiment was repeated for photosynthesis, but this time a radioactive tag was put on the oxygen in CO 2. 2. Using the blocks provided and the sticker, model what you predict the appearance of the results would be. Your model must include a “tag” to indicate the oxygen isotope on the left side of the arrow and where it ends up on the right side of the arrow. 3. Observe the blocks that represent carbon and hydrogen and what happens to them in the reaction.

9. Analysis TURN IN YOUR WORK